To study risk factors for placental abruption, we used data from the Kaiser Permanente Birth Defects Study, which is conducted by the National Institute of Child Health and Human Development between 1974 and 1978. This was a prospective cohort study which included 30,681 members of the Kaiser Permanente Health Plan who registered for prenatal care at one of 13 Kaiser facilities in Northern California. Demographic and social information was collected from these women at the first prenatal visit, and data about pregnancy complications and neonatal outcomes were obtained by review of the patient records. In our analysis, we found that important maternal risk factors for abruption included maternal age, less than 12 years of education, and cigarette smoking. Abruption was also significantly associated with intrauterine growth retardation and fetal malformations, and particularly with congenital heart defects.